A conventional voltage controlled oscillator is one of the important devices that have been widely used in a communication system. Especially, an LC voltage controlled oscillator is an indispensable device in a recent wireless mobile communication because of its original characteristics of low noise. FIG. 1 shows a circuit of the conventional LC differential voltage controlled oscillator.
In a recent wireless mobile communication field, especially, in a next generation of a second generation thereof, a voltage controlled oscillator that is usable in a wide bandwidth and a multiple bandwidth is required. A conventional method for expanding a bandwidth of a voltage controlled oscillator includes a method using a Yttrium Iron Garnet(YIG)-tuned oscillator, a method using a switching device including multiple inductors or capacitors, a method using an off-chip variable capacitor that operates with a high voltage applied thereto, and the like.
However, the YIG-tuned oscillator capable of being used in a wide bandwidth requires a high voltage and is not appropriate for a wireless mobile communication terminal due to a considerably large size of a circuit.
Further, the method using a switching device including multiple inductors or capacitors has a problem in that multiple inductors are inserted in one chip or multiple input signals are needed to control the switching device. And, in case of the method using an off-chip variable capacitor, a high voltage is required and, particularly, a high integration thereof is difficult to achieve.